1. Field
The disclosed embodiments relate generally to the field of communications, and more specifically to methods and apparatus for operation of a forward link acknowledgement channel.
2. Background
The field of communications has many applications including, e.g., paging, wireless local loops (WLL), Internet telephony, and satellite communication systems. An exemplary application is a cellular telephone system for mobile subscribers. Modern communication systems designed to allow multiple users to access a common communications medium have been developed for such cellular systems. These communication systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), or other multiple access techniques known in the art. These multiple access techniques decode and demodulate signals received from multiple users, thereby enabling simultaneous communication among multiple users and allowing for a relatively large capacity for the communication systems.
In the CDMA system, the available spectrum is shared efficiently among a number of users, and techniques such as soft handoff are employed to maintain sufficient quality to support delay-sensitive services (such as voice) without wasting a lot of power. More recently, systems that enhance the capacity for data services have also been available. These systems provide data services by using higher order modulation, faster power control, faster scheduling, and more efficient scheduling for services that have more relaxed delay requirements. An example of such a data-services communication system is the high data rate (HDR) system that conforms to the Telecommunications Industry Association/Electronic Industries Alliance (TIA/EIA) cdma2000 High Data Rate Air Interface Specification IS-856, January 2002 (the IS-856 standard).
In a CDMA system, data transmission occurs from a source device to a destination device. The destination device receives the data transmission, demodulates the signal, and decodes the data. As part of the decoding process, the destination device performs the Cyclic Redundancy Code (CRC) check of the data packet to determine whether the packet was correctly received. Error detection methods other than the use of CRC, e.g., energy detection, can also be used in combination with or instead of CRC. If the packet was received with an error, the destination device transmits a negative acknowledgement (NAK) message on its acknowledgement (ACK) channel to the source device, which responds to the NAK message by retransmitting the packet that was received with an error.
Transmission errors may be particularly acute in applications with a low signal quality (e.g., low bit energy-to-noise power spectral density ratio (Eb/No)). In this situation, a conventional data retransmission scheme, such as Automatic Repeat Request (ARQ), may not meet (or may be designed not to meet) the maximum bit error rate (BER) required for the system operation. In such a case, combining the ARQ scheme with an error correction scheme, such as a Forward Error Correction (FEC), is often employed to enhance performance. This combination of ARQ and FEC is generally known as Hybrid ARQ (H-ARQ).
After transmitting a NAK, the destination device receives the data transmission and retransmission, demodulates the signal, and separates the received data into the new packet and the retransmitted packet. The new packet and the retransmitted packet need not be transmitted simultaneously. The destination device accumulates the energy of the received retransmitted packet with the energy already accumulated by the destination device for the packet received with an error. The destination device then attempts to decode the accumulated data packet. However, if the packet frame is initially transmitted with insufficient energy to permit correct decoding by the destination device, as described above, and is then retransmitted, the retransmission provides time diversity. As a result, the total transmit energy of the frame (including retransmissions) is lower on average. The combined symbol energy for both the initial transmission and retransmission(s) of the frame is lower than the energy that would have been required to transmit the frame initially at full power (i.e., at a power level that was sufficient on its own to permit correct decoding by the destination device) on average. Thus, the accumulation of the additional energy provided by the subsequent retransmissions improves the probability of a correct decoding. Alternately, the destination device might be able to decode the retransmitted packet by itself without combining the two packets. In both cases, the throughput rate can be improved since the packet received in error is retransmitted concurrently with the transmission of the new data packet. Again, it should be noted that the new packet and the retransmitted packet need not be transmitted simultaneously.
In the reverse link (i.e., the communication link from the remote terminal to the base station), the reverse supplemental channel (R-SCH) is used to transmit user information (e.g., packet data) from a remote terminal to the base station, and to support retransmission at the physical layer. The R-SCH may utilize different coding schemes for the retransmission. For example, a retransmission may use a code rate of ½ for the original transmission. The same rate ½ code symbols may be repeated for the retransmission. In an alternative case, the underlying code may be a rate ¼ code. The original transmission may use ½ of the symbols and the retransmission may use the other half of the symbols. An example of the reverse link architecture is described in detail in U.S. patent application No. 2002/0154610, entitled “REVERSE LINK CHANNEL ARCHITECTURE FOR A WIRELESS COMMUNICATION SYSTEM” assigned to the assignee of the present application.
In a CDMA communication system, and specifically in a system adapted for packetized transmissions, congestion and overloading may reduce the throughput of the system. The congestion is a measure of the amount of pending and active traffic with respect to the rated capacity of the system. System overload occurs when the pending and active traffic exceeds the rated capacity. A system may implement a target congestion level to maintain traffic conditions without interruption, i.e., to avoid overloading and underloading of resources.
One problem with overloading is the occurrence of delayed transmission responses. An increase in response time often leads to application level timeouts, wherein an application requiring the data waits longer than the application is programmed to allow, producing a timeout condition. Applications will then needlessly resend messages on timeouts, causing further congestion. If this condition continues, the system might reach a condition where it can service no users. One solution (used in HDR) for this condition is congestion control. Another solution (used in cdma2000) is proper scheduling.
The level of congestion in a system may be determined by monitoring the data rates of pending and active users, and the received signal strength required to achieve a desired quality of service. In a wireless CDMA system, the reverse link capacity is interference-limited. One measure of the cell congestion is the total amount of noise over the level of the thermal noise at a base station (referred to hereafter as the “rise over thermal” (ROT)). The ROT corresponds to the reverse link loading. A loaded system attempts to maintain the ROT near a predetermined value. If the ROT is too high, the range of the cell (i.e., the distance over which the base station of the cell can communicate) is reduced and the reverse link is less stable. The range of the cell is reduced because of an increase in the amount of transmit energy required to provide a target energy level. A high ROT also causes small changes in instantaneous loading that result in large excursions in the output power of the remote terminal. A low ROT can indicate that the reverse link is not heavily loaded, thus indicating that available capacity is potentially being wasted.
However, operating the R-SCH with H-ARQ may require that the initial transmission of an R-SCH frame not be power controlled very tightly to meet the ROT constraints. Therefore, the delivered signal-to-noise ratio (SNR) on the initial transmission of an R-SCH frame can be below the level sufficient to permit correct decoding of the received data packet. This condition can result in a NAK message being transmitted over the forward link ACK channel.
Accordingly, from the discussion above, it should be apparent that there is a need in the art for an apparatus and method that enables efficient operation of the forward link ACK channel.